2019PASP..131i4203W

Cepheid variable stars calibrate the cosmic distance ladder and exhibit large-amplitude pulsations that provide interesting hydrodynamical insights into stellar atmospheres. Here, we investigate velocities of different spectral line species (ionization levels, elements) to provide new detail into the complex velocity structure of the particularly large-amplitude Cepheid X Cygni, P_puls = 16.3 d. This is done against the backdrop of studies aimed at understanding the projection factor for Baade-Wesselink-type distance methods, as well as the timeseries velocimetry being gathered by the Gaia satellite. We analyze and compare velocities measured by cross-correlation using thousands of optical metallic lines, as well as individual line velocities of hydrogen (Balmer and Paschen lines), oxygen, ionized silicon, potassium, sodium, and ionized calcium. We compare individual line velocity curves to a reference velocity curve based on cross-correlation and discuss observed differences as a function of pulsation phase. This purely empirical approach yields detailed insights into the dependence of certain lines on NLTE effects, shocks, and phase lags in velocity space, which can be used as strong constraints for hydrodynamical modeling. Of particular importance is the velocity curve of the Ca II IR triplet, which dominates the Gaia radial velocities of F, G, and K type variable stars.